Abstract:

An implantable flat blood pressure sensing cuff structure and an
implantable blood pressure monitoring device use a first portion of the
cuff structure that comprises a sidewall that extends from a surface and
contains a pressure sensor, and a second portion of the cuff structure
that is configured to overlie and be removably relative to the first
portion. The first and second portions of the cuff structure are
configured to provide an aperture extending transversely through the cuff
structure for receiving a blood vessel therein generally sandwiched
between the first portion and the second portion when the second portion
is positioned on the first portion such that the pressure sensor is
operative to detect vessel expansion and contraction.

Claims:

1. An implantable flat blood pressure sensing cuff structure comprising:a
first portion that comprises a sidewall that extends from a surface and
contains a pressure sensor; anda second portion that is configured to
overlie and be removably relative to the first portion, the first and
second portions being configured to provide an aperture extending
transversely through the cuff structure for receiving a blood vessel
therein generally sandwiched between the first portion and the second
portion when the second portion is positioned on the first portion such
that the pressure sensor is operative to detect vessel expansion and
contraction.

2. The cuff structure of claim 1, further comprising a flexible membrane
that retains a fluid within the sidewall of the first portion, the
flexible membrane being positioned to interface with the blood vessel to
respond to expansion and contraction of said blood vessel.

3. The cuff structure of claim 2, wherein the pressure sensor comprises a
capacitive pressure sensor located inside the fluid between the sensing
membrane and a substrate on which the first portion is situated.

4. The cuff structure of claim 2, wherein the pressure sensor comprises a
piezoresistive pressure sensor located inside the fluid between the
sensing membrane and a substrate on which the first portion is situated.

5. The cuff structure of claim 2, wherein the pressure sensor is coupled
to a circuit board that comprises and an integrated circuit chip.

6. The cuff structure of claim 5, wherein the pressure sensor is mounted
on the integrated circuit chip, which is mounted on the circuit board.

7. The cuff structure of claim 6, wherein the pressure sensor is mounted
on the circuit board, and wherein the integrated circuit chip is mounted
on the circuit board in the middle of an RF powering coil mounted on the
circuit board.

8. The cuff structure of claim 1, wherein a layer of rigid material is
disposed within the second portion and in overlying relation relative to
the pressure sensor to isolate the environmental variation from being
coupled to the pressure sensor.

9. The cuff structure of claim 1, further comprising a retaining mechanism
for retaining the relative position between the first portion and the
second portion.

10. The cuff structure of claim 9, wherein the retaining mechanism is a
rigid strap that covers at least some of a top surface of the second
portion and at least a bottom surface of a substrate coupled to the first
portion.

11. An implantable blood pressure monitoring device comprising:a
substrate; anda flat blood pressure sensing cuff structure situated on
the substrate, the cuff structure comprising;a first portion that
comprises a sidewall that extends from a surface and contains a pressure
sensor; anda second portion that is configured to overlie and be
removably relative to the first portion, the first and second portions
being configured to provide an aperture extending transversely through
the cuff structure for receiving a blood vessel therein generally
sandwiched between the first portion and the second portion when the
second portion is positioned on the first portion such that the pressure
sensor is operative to detect vessel expansion and contraction.

12. The device of claim 11, wherein the cuff structure further comprises a
flexible membrane that retains a fluid within the sidewall of the first
portion, the flexible membrane being positioned to interface with the
blood vessel to respond to expansion and contraction of said blood
vessel.

13. The device of claim 12, wherein the pressure sensor comprises a
capacitive pressure sensor located inside the fluid between the sensing
membrane and a substrate on which the first portion is situated.

14. The device of 12, wherein the pressure sensor comprises a
piezoresistive pressure sensor located inside the fluid between the
sensing membrane and a substrate on which the first portion is situated.

15. The device of claim 12, wherein the substrate includes a circuit board
that comprise an integrated circuit chip and wherein the pressure sensor
is coupled to the circuit board.

16. The device of claim 15, wherein the pressure sensor is mounted on the
integrated circuit chip, which is mounted on the circuit board.

17. The device of claim 15, further comprising:an RF powering coil mounted
to the circuit board for receiving a wireless RF powering signal from a
remote RF power source;circuitry configured to convert the RF powering
signal for powering the integrated circuit chip and the pressure sensor;
andan antenna for wirelessly transmitting sensor information from the
pressure sensor to a receiver.

18. The device of claim 17, wherein the pressure sensor is mounted on the
circuit board, and wherein the integrated circuit chip is mounted on the
circuit board within in the RF powering coil.

19. The device of claim 11, wherein the cuff structure further comprises a
layer of rigid material disposed within the second portion and in
overlying relation relative to the pressure sensor to isolate the
environmental variation from being coupled to the pressure sensor.

20. The device of claim 11, wherein the cuff structure further comprises a
retaining mechanism for retaining the relative position between the first
portion and the second portion.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001]This application is entitled to the benefit of U.S. Provisional
Patent Application Ser. No. 61/242,835, filed on Sep. 16, 2009, which is
incorporated herein by reference.

FIELD OF THE INVENTION

[0003]The invention relates to monitoring devices, and more particularly
to an implantable blood monitoring device.

BACKGROUND OF THE INVENTION

[0004]A variety of animals are used for biomedical research. As but one
example, genetically engineered mice may be considered one of the most
important animal models for advanced biomedical research because these
mice share a high degree of homology to humans with completely sequenced
genetic information. Furthermore, mice can be reproduced quickly, which
is crucial for system biology research, where several generations of mice
are typically required to obtain the desirable strains with targeted
genetic sequences.

[0005]However, the small blood vessel size of mice, approximately 200
μm in diameter for major arteries, introduces significant challenges
for in vivo blood pressure monitoring on free roaming animals. It is
difficult to apply many previously developed types of blood pressure
sensing cuffs in mice for in vivo real-time monitoring. Furthermore,
complexities often arise due to significantly increased complexity in
fabrication, packaging, and implant procedure.

[0006]In view of these concerns, there is a need for an implantable blood
monitoring device for effective in vivo real-time monitoring.

SUMMARY OF THE INVENTION

[0007]An implantable flat blood pressure sensing cuff structure in
accordance with an embodiment of the invention comprises a first portion
that comprises a sidewall that extends from a surface and contains a
pressure sensor and a second portion that is configured to overlie and be
removably relative to the first portion. The first and second portions
are configured to provide an aperture extending transversely through the
cuff structure for receiving a blood vessel therein generally sandwiched
between the first portion and the second portion when the second portion
is positioned on the first portion such that the pressure sensor is
operative to detect vessel expansion and contraction.

[0008]An implantable blood pressure monitoring device in accordance with
an embodiment of the invention comprises a substrate and a flat blood
pressure sensing cuff structure situated on the substrate. The cuff
structure comprises a first portion that comprises a sidewall that
extends from a surface and contains a pressure sensor, and a second
portion that is configured to overlie and be removably relative to the
first portion. The first and second portions are configured to provide an
aperture extending transversely through the cuff structure for receiving
a blood vessel therein generally sandwiched between the first portion and
the second portion when the second portion is positioned on the first
portion such that the pressure sensor is operative to detect vessel
expansion and contraction.

[0009]Other aspects and/or advantages of the present invention will become
apparent from the following detailed description, taken in conjunction
with the accompanying drawings, illustrated by way of example of the
principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view of an implantable blood pressure
monitoring device in accordance with an embodiment of the invention.

[0011]FIG. 2 is a cross-sectional view of the implantable blood pressure
monitoring device.

[0012]FIG. 3 is a diagram depicting an example embodiment of the
implantable blood pressure monitoring device, which is resting on a dime
to help demonstrate its small size.

[0013]FIG. 4 depicts an example of circuitry that is configured to provide
automatic offset cancellation for a sensor, which can be included in the
implantable blood pressure monitoring device, in accordance with an
embodiment of the invention.

[0014]FIG. 5 depicts a single-ended capacitive pressure sensor, which can
be included in the implantable blood pressure monitoring device, in
accordance with an embodiment of the invention.

[0015]FIG. 6 depicts another example of circuitry that is configured to
provide capacitance offset cancellation for a sensor, which can be
included in the implantable blood pressure monitoring device, in
accordance with an embodiment of the invention.

[0016]FIG. 7 depicts an example system that can implement adaptive RF
powering, which can be included in the implantable blood pressure
monitoring device, in accordance with an embodiment of the invention.

[0017]FIG. 8 depicts another example of a system that can implement
adaptive RF powering, which can be included in the implantable blood
pressure monitoring device, in accordance with an embodiment of the
invention.

[0018]FIG. 9 is a timing diagram illustrating the operation of the system
shown in FIG. 8 in accordance with an embodiment of the invention.

[0019]FIG. 10 depicts an example of a blood pressure monitoring system
implanted in an animal in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0020]It will be readily understood that the components of the embodiments
as generally described herein and illustrated in the appended figures
could be arranged and designed in a wide variety of different
configurations. Thus, the following more detailed description of various
embodiments, as represented in the figures, is not intended to limit the
scope of the present disclosure, but is merely representative of various
embodiments. While the various aspects of the embodiments are presented
in drawings, the drawings are not necessarily drawn to scale unless
specifically indicated.

[0021]The described embodiments are to be considered in all respects only
as illustrative and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by this detailed
description. All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their scope.

[0022]Reference throughout this specification to features, advantages, or
similar language does not imply that all of the features and advantages
that may be realized with the present invention should be or are in any
single embodiment. Rather, language referring to the features and
advantages is understood to mean that a specific feature, advantage, or
characteristic described in connection with an embodiment is included in
at least one embodiment. Thus, discussions of the features and
advantages, and similar language, throughout this specification may, but
do not necessarily, refer to the same embodiment.

[0023]Furthermore, the described features, advantages, and characteristics
of the invention may be combined in any suitable manner in one or more
embodiments. One skilled in the relevant art will recognize, in light of
the description herein, that the invention can be practiced without one
or more of the specific features or advantages of a particular
embodiment. In other instances, additional features and advantages may be
recognized in certain embodiments that may not be present in all
embodiments of the invention.

[0024]Reference throughout this specification to "one embodiment," "an
embodiment," or similar language means that a particular feature,
structure, or characteristic described in connection with the indicated
embodiment is included in at least one embodiment. Thus, the phrases "in
one embodiment," "in an embodiment," and similar language throughout this
specification may, but do not necessarily, all refer to the same
embodiment.

[0025]FIGS. 1 and 2 depict an example of an implantable blood pressure
monitoring device 10 in accordance with an embodiment of the invention.
FIG. 1 depicts a perspective view of the device and FIG. 2 depicts a
cross sectional view. The device 10 includes a substantially flat
implantable blood pressure sensing cuff structure 12 that can be utilized
for in vivo blood pressure monitoring. The cuff structure 12 includes
opposing cuff portions 14 and 16 that are dimensioned and configured to
sandwich a blood vessel 18 therebetween for blood pressure monitoring.
The monitoring cuff structure 12 can be fabricated through a silicone
molding technology to have minimal (or very small) restriction on the
blood vessel.

[0026]In FIGS. 1 and 2, the cuff portion 14 is illustrated as the top
portion of the cuff structure 12 and the other cuff portion 16 is
illustrated as the bottom portion of the cuff structure (although top and
bottom do not imply a necessary relationship during use). The top cuff
portion 14 can be formed by a silicone layer. As shown in FIG. 2, a sheet
17 of rigid material, which may be conductive material (e.g., a 75
μm-thick stainless steel sheet), can be embedded within the top cuff
portion 14 for shielding environmental effects, such as due to
contractions or movements of surrounding tissues. In an embodiment,
another sheet of rigid material, similar to the sheet 17, may be placed
under the substrate 22 to achieve the same shielding purpose. The elastic
sidewalls of the bottom cuff portion 16 (e.g., of Silicone) extend
upwardly from a substrate to terminate in a top surface of the bottom
cuff portion. As illustrated in FIG. 2, a spacer 19 of a flexible
material (e.g., silicone) can be positioned between the top cuff portion
14 and the bottom cuff portion 16, similar to a gasket. The spacer 19 can
be dimensioned as to be smaller than the blood vessel 18 diameter to help
ensure a proper contact between the cuff structure and the blood vessel,
while also controlling the pressure applied to the blood vessel. The cuff
structure 12 can be used for mice as well as for other animals, with the
size of the structure being determined according to the animal for which
the implant is intended to be utilized.

[0027]The relative superimposed overlying relationship of the top cuff
portion 14 and the bottom cuff portion 16 can be maintained by a
retaining mechanism 20, such as a clamp or surgical sutures. In the
example of FIG. 2, the retaining mechanism 20 is depicted as a resilient
wire clamp (e.g., of stainless steel) that includes an arm that is
configured to apply a retaining force that urges the top cuff portion 14
toward the bottom cuff portion 16 and the substrate 22 on which the cuff
structure 12 is situated. In other embodiments, the retaining mechanism
may be a thin rigid strap, which may be made of metallic material. The
retaining strap can be applied to the cuff structure in the same way as
the wire clamp shown in FIG. 2, but made to cover all or most of the top
surface of the top cuff portion 14 and partial bottom surface of the
substrate 22, which coincides with the IC Chip 30. The retaining strap
can isolate the environmental variation and tissue growth from being
coupled to the MEMS sensor. Those skilled in the art will understand
other means (e.g., tabs, latches, clips or the like) that could be
utilized for holding the cuff structure in a desired position after the
blood vessel has been positioned therein as to extend through a laterally
extending aperture in the sidewall of the cuff structure 12.

[0028]By way of example, a juncture 26 between top cuff portion 14 and the
bottom cuff portion 16 is configured to provide an opening/closing point
to provide access to the interior of the cuff 12 for vessel insertion.
Such access can be had by removing the retaining clamp 20 and urging the
top cuff portion away from the bottom cuff portion 16 and the substrate
22, such as in the direction of arrow 28. For instance, during
implantation, a top arm of the clamp 20 is first lifted up and the top
cuff portion is also moved in the direction indicated by the arrow 28 to
create an opening at the juncture 26. The blood vessel 18 can then be
inserted through the opening/closing point at the juncture 26 and the top
arm of the clamp can be released to re-engage and secure the cuff
structure 12 in position around the blood vessel, such as shown in FIG.
2. It will be appreciated that the cuff structure 12 thus substantially
simplifies the implant procedure.

[0029]As further shown in FIG. 2, a thin flexible sensing membrane 24,
which may be made of silicone or other flexible material, extends across
the top edge of the bottom cuff portion 16 to enclose a cavity within the
bottom cuff portion. An insulating fluid 29, such as low viscosity oil
like fluid (e.g., silicone oil or silicone gel), can occupy the cavity
along with a microelectromechanical systems (MEMS) sensor 40. Thus, the
MEMS sensor is immersed in the fluid. The MEMS sensor can be implemented
as a capacitive pressure sensor or a piezoresistive pressure sensor. The
blood vessel 18 is positioned above the MEMS sensor, such as near the
center of the cuff structure 12. Thus, the flexible membrane is
positioned to interface with the blood vessel to respond to expansion and
contraction of said blood vessel. The MEMS sensor is mounted to an
integrated circuit (IC) chip 30, which can be mounted to the substrate
22, such as a flexible printed circuit board. The IC chip can constitute
a bottom part of the cuff structure 12 from which the sidewalls of the
cuff structure extend. The combination of the MEMS sensor and the IC chip
on which the MEMS sensor is mounted will be referred to herein as a
MEMS/IC pressure sensing module.

[0030]In operation, the expansion and contraction of the blood vessel 18
in the cuff structure 12, which is caused by the blood pressure in the
blood vessel, will be detected by the MEMS/IC pressure sensing module
through the coupling of the insulating fluid in the cavity, which is
defined by at least the bottom cuff portion 16 and the sensing membrane
24. Thus, the blood pressure information can be measured by the MEMS/IC
pressure sensing module as blood pressure waveform or signal.

[0031]By way of example, the fabricated flat cuff structure 12 can be
dimensioned and configured to have a miniature form factor, which is
sufficiently small for implant in small laboratory animals such as mice.
Various dimensions can be implemented according to application
requirements. The measured waveform represents a down-scaled version of
the blood pressure in the blood vessel 18, which can be calibrated by
using a tail-cuff-based blood pressure measurement apparatus. The
stainless steel sheet 17 embedded in the top cuff portion 14 further
strengthens the structural rigidity of the cuff structure. The stainless
steel sheet 17 together with the MEMS/IC pressure sensing module further
can effectively suppress environmental variations effect on the cuff
performance. The flexible PCB substrate 22 contains discrete components
36 and a radio frequency (RF) powering coil 38. The discrete components
36 and the RF power coil 38 do not necessarily need to be adjacent to the
flat cuff structure 12. The discrete components 36 and the RF power coil
38 can be placed away and connected to the cuff structure 12 by flexible
wires according to application requirements. The IC chip 30 can be also
placed in the middle of the RF powering coil 38 along with other discrete
components 36, thus allowing a large IC chip to be employed for the
design without enlarging the dimension of the blood pressure measuring
device 10, which includes the cuff structure 12, the MEMS sensor 40, and
the IC chip 30. A large IC chip can offer an increased sensing
capability, including sensing animal body temperature and other
bio-potential signals such as electrocardiogram (EKG),
electroencephalography (EEG), and electroencephalography (EMG). Circular
blood pressure sensing cuff filled by insulating fluid, such as low
viscosity oil like fluid (e.g., silicone oil or silicone gel), with an
immersed MEMS pressure sensor was shown to be adequate for in vivo blood
pressure sensing for large animals, having major arteries size around 1
mm. However, implant experiments showed that circular cuffs failed to
work for small animals having small arteries size. The flat blood
pressure sensing cuff structure 12 is ideal for small animal in vivo
blood pressure monitoring.

[0032]FIG. 3 is a diagram depicting an example embodiment of the
implantable blood pressure monitoring device 10. In this example, the
device 10 is resting on a dime to help demonstrate its small size.

[0033]In view of the foregoing, it will be appreciated that the
implantable blood pressure monitoring device 10 thus can be designed to
be small, light weight, tissue compatible and easily installable during
implantation. Associated circuitry (e.g., as shown and described herein)
can further be utilized to measure small fraction of changes of the blood
vessel through the fluid 29 coupling to the MEMS pressure sensor 40.

Automatic Off-Set Cancellation Circuitry

[0034]Circuitry 100 for performing automatic offset cancellation will be
described with respect to examples depicted in FIGS. 4, 5 and 6. Offset
adjustment is useful for long term implants as the body environment in
which the implant resides may change with the host's health and other
conditions. Thus, the circuitry 100 may be used in the implantable blood
pressure monitoring device 10. However, the circuitry 100 may be used in
other implantable devices or other devices where automatic offset
cancellation may be useful.

[0035]FIG. 4 depicts an example of the circuitry 100 that is configured to
provide automatic offset cancellation for a sensor in accordance with an
embodiment of the invention. A regulated stimulation voltage VS is
supplied via a pair of switches based on switch controls signals Φ1
and Φ2 to provide VS to drive respective capacitors Cs and CR. The
capacitor CS is connected between the input stimulation voltage VS and an
inverting input of an amplifier 102. The capacitor CS has a capacitance
that varies as a function of pressure being sensed thereby. For instance,
the capacitor CS can be implemented as a capacitive pressure sensor shown
and described herein. In one example embodiment, the capacitor CS can be
implemented as a single-ended capacitive pressure sensor, such as shown
in FIG. 5.

[0036]The input signal is also provided to a variable reference capacitor
CR coupled between the input stimulation voltage VS and a non-inverting
input of the amplifier 102. The capacitor CR can be implemented as a
digitally controlled array of capacitors connected in parallel. Each of
the capacitors in the array CR is connected between the non-inverting
input node and ground or the input stimulation voltage via digitally
controlled switches D0-Dn, where n is a positive integer and 2n
denotes the number of unit capacitors.

[0037]The amplifier 102 is configured to operate as a
capacitance-to-voltage converter that provides a differential output at
Voutput+ and Voutput-. Feedback capacitors C1 are connected between each
input-output pair of the amplifier 102. A range detector 104 is connected
to the differential output to provide a range signal to a logic block
106. As an example, the range detector 104 may be a comparator with
designed hysteresis characteristics corresponding to the input detection
range. The logic block provides an off-set cancellation enable signal (an
inverted version thereof) to control calibration of the capacitor array
CR. During the initial phase of the circuit operation, the digitally
controlled reference capacitor array CR at the amplifier input can be
enabled by the logic 106 to cycle through its range of capacitances in
response to a switch control signal ΦMST. The logic or other
circuitry further can control the cycling through capacitors to find a
reference capacitance value that is closely matched to a nominal
capacitance of CS. Once the capacitor array CR is within the desired
range of the nominal CS, the logic 106 can disable the cycling through
the capacitance values.

[0038]It will be appreciated that this technique allows a single-ended
capacitive pressure sensor with a wide range of nominal capacitance value
to be employed, thus greatly simplifying MEMS fabrication process and
relaxing tolerance requirements, and also effectively suppressing the
output offset voltage. As one example, the unit capacitor in the
capacitor array CR can be set to 20 fF, such as can be implemented as a
poly-poly capacitor according to a selected fabrication process (e.g.,
1.5 μm CMOS process). Those skilled in the art will appreciate various
processes that can be utilized.

[0039]FIG. 6 depicts another example of circuitry 200 that is configured
to provide capacitance offset cancellation for a single-ended capacitive
sensor CS (see, e.g., FIG. 5) in accordance with an embodiment of the
invention. Similar to the example of FIG. 4, a stimulation voltage VS is
supplied to the circuitry 200 by switching a pair of switches, such that
the input voltage can alternate between VS and electrical ground based on
switch control signals Φ1 and Φ2.

[0040]In the example of FIG. 6, the circuitry 200 is implemented as a
fully differential correlated-double-sampling (CDS)
capacitance-to-voltage (C/V) converter. The input signal is provided to a
variable capacitor (e.g., a MEMS capacitive pressure sensor) CS that is
connected between the supply input and an inverting input of an amplifier
A1. The input signal is also provided to a variable reference capacitor
(e.g., digitally controlled reference capacitor array) CR coupled between
the input supply and a non-inverting input of the amplifier A1. Input
common-mode feedback (ICMFB) can be implemented relative to the input
signal at the first stage 202. Output common-mode feedback (OCMFB) can
also be implemented for each of the stages 202 and 204.

[0041]The converter 200 includes a multi-stage arrangement of amplifier
stages 202 and 204. The first stage 202 operates as a
capacitance-to-voltage converter and the second stage 204 is a gain stage
configured to amplify the differential output from the first stage for
subsequent processing. For instance, the output of the second gain stage
204 can be converted to a corresponding digital signal that can be sent
wirelessly to a data acquisition unit, such as shown and described
herein.

[0042]Feedback capacitors C1 are connected between each input-output pair
of the first stage amplifier A1. Switches can also be connected in
parallel with each of the capacitors C1 for resetting the first stage in
response to a reset signal ΦRST. A range detector 206 is connected to
the differential output of the first stage amplifier A1 to provide an
off-set cancellation enable signal (an inverted version thereof) to
control calibration of the capacitor array CR. For instance, the off-set
cancellation enable signal can be provided to enable/disable a counter
208 that controls switches associated with each of the capacitors in the
capacitive array CR. For instance, the counter can be an eight-bit
counter, although other numbers of bits can be used according to the size
of the reference capacitor array CR.

[0043]The output of the first stage 202 is provided as the input to the
second stage 204 through capacitors CH, which are coupled between the
outputs of the first stage and the inputs of the second stage amplifier
A2. The second stage also includes feedback capacitors C11 selected to
provide a desired amount of gain for the differential input signal. An
arrangement of switches is configured to control operation of the second
stage 204 for providing the amplified output signal.

[0044]By way of example, during the initial phase of the circuit
operation, the digitally controlled reference capacitor array CR at the
amplifier input can be cycled through its range of capacitances based on
an output of the counter 208 to find a reference capacitance value CR
that substantially matches a nominal capacitance of the single-ended
capacitive sensor CS. Once the capacitor array CR is within the desired
range of the single-ended capacitive sensor CS, the range detector 206
disables the counter 208.

[0045]The circuitry 200 thus allows the single-ended capacitive pressure
sensor CS having a wide range of nominal capacitance value to be
employed. As a result, the MEMS fabrication process for the single-ended
capacitive sensor CS can be simplified and the tolerance requirements can
be relaxed. Additionally, the offset cancellation function effectively
suppresses the output offset voltage.

Adaptive RF Powering

[0046]In classical RF powering designs, the relative position of the
internal coil is usually fixed with respect to the external coil. The
case of an implant in an untethered laboratory animal differs from these
typical design scenarios in that the inductive coupling factor (k), and
thus the power coupled into the implant, can vary drastically over the
operating region as the internal coil tilts and changes its position with
respect to the stationary external coil. Adaptive RF powering, as
described herein, can help prevent power variations from damaging on-chip
components or distorting sensitive biosignals and will save the total
power of the system. The adaptive RF powering can be used in any mobile
powering systems, such as including industrial or medical applications,
including the implantable blood pressure monitoring device 10.

[0047]The adaptive RF powering will be described with respect to FIGS.
7-10. The adaptive RF powering can be implemented as a closed loop
control mechanism that affords a substantially constant received power
level.

[0048]FIG. 7 depicts an example system 300 that can implement adaptive RF
powering in accordance with an embodiment of the invention. While the
example of FIG. 7 demonstrates the adaptive RF powering in the context of
an implantable sensor system 300, it will be appreciated that the
adaptive RF powering can be implemented in a variety of other contexts,
including industrial and medical applications.

[0049]In FIG. 7, the system 300 includes an adaptive RF-to-DC converter
302 that receives a wireless RF power signal via an antenna 304. The
power signal may also carry additional information (e.g., control
instructions) modulated on a carrier. Responsive to the RF power signal,
the power converter 302 provides a clock signal (CLK) and a regulated
voltage (VDD) to a system configuration control unit 306. The adaptive
RF-to-DC power converter 302 is configured to provide sufficient and
stable energy to the system 300, such as a system that can be implanted
in an untethered animal.

[0050]The control unit 306 provides power and control signals to
associated circuitry, which in the example of FIG. 7 include interface
electronics 308 and an analog-to-digital converter (ADC) 310. A sensor
312 is connected to provide a sensor signal to the interface electronics
308. For example, the sensor can be a capacitive pressure sensor (see,
e.g., FIG. 5) and the interface electronics can be implemented to include
conversion circuitry, such as shown and described with respect to FIG. 4
or FIG. 6.

[0051]The interface electronics 308 thus can provide an analog output
signal to the ADC 310. The ADC 310 can convert the analog signal to a
corresponding digital signal that is combined with digital
control/feedback information from the control unit 306. The combined
digital information can be provided to a transmitter (e.g., an FSK
transmitter) 314 that wirelessly transmits the signal via an antenna 316.
The combined information thus can include sensor data from the sensor 312
and RF power data from the control unit 306. The RF power data can
indicate a level of the input RF power received via the antenna 304, such
as corresponding to a quantized power level.

[0052]The RF power data can be utilized to adaptively adjust the RF power
provided by a source of RF power to the system 300. For example, the
input voltage for a power amplifier can be adjusted in response to the RF
power data so that the received power level at the system 300 remains
substantially constant. Thus, the RF power data can provide closed loop
feedback for controlling the RF power being delivered to a substantially
constant level.

[0053]RF powering has been widely used for biomedical implants, where both
transmitting and receiving units are properly placed at a fixed distance
from each other with a constant RF power coupling coefficient. However,
the receiving unit can also be implanted inside a freely moving
laboratory animal resulting in a continuously changing RF power coupling.
RF powering can also be used to eliminate the need of a battery, thus
substantially reducing the overall size and weight of the system 300.
Furthermore, a miniature RF coil can be used due to low power budget of
the integrated electronics, further minimizing the total system size and
weight.

[0054]FIG. 8 depicts another example of a system 400 that can implement
adaptive RF powering in accordance with an embodiment of the invention.
The system 400 demonstrates a remote power source 402 that is configured
to provide RF power to a moveable apparatus 404, such as an implant
within an untethered animal. The power source 402 can be implemented as a
power amplifier (e.g., a class-E amplifier) that provides an output
signal to a coil L1 that is configured to provide RF powering. For the
example of a class-E power amplifier, the output power varies based on an
input supply voltage VDD and a duty cycle of a switching signal. Given a
constant duty cycle, the power source 402 controls the output power as a
function of the input supply voltage VDD. Thus, in the system 400,
circuitry is provided to adjust VDD (e.g., discretely or continuously) in
response to RF power data provided by the moveable apparatus 404. A
capacitor C1 can be connected between the amplifier and the coil L1 to
provide the desired alternating power signal to the coil L1.

[0055]The RF power signal can be received at the apparatus 404 via a coil
antenna L2. The inductive coupling results in current that is converted
to a voltage by power conversion circuitry 406, such as a voltage
doubler. The conversion circuitry 406 can thus provide a differential
voltage, indicated at VHIGH and VLOW, to an arrangement of voltage
regulation circuitry 408. The voltage regulation circuitry 408 can in
turn provide regulated output voltage at or near corresponding levels
needed for operation of the associated circuitry according to the
received RF power.

[0056]An RF power level sensor 410 provides a digital signal indicative of
the received RF power level. In the example of FIG. 8, the RF power level
sensor 410 can be coupled to monitor the VLOW voltage provided by the
conversion circuitry 406. The sensed voltage is mirrored through an
arrangement of transistors to provide a corresponding voltage across a
sense resistor. The voltage is provided to an input of a comparator (or
threshold detector), which compares the voltage to a reference voltage
(VR). The comparator in turn provides a digital output signal as a
quantized one-bit signal representing the power level of the RF input
power relative to the reference voltage VR. For example, the quantized
power signal can be generated continuously while the apparatus 404
receives RF powering from the source 402.

[0057]An input supply voltage from the regulator 408 is provided to power
a variable capacitor (e.g., a MEMS capacitor pressure sensor) 412. A
capacitance-to-voltage converter and sampling circuitry 414 converts the
capacitance to a corresponding analog voltage signal. The converter and
sampling circuitry can be implemented as shown and described with respect
to FIGS. 4 and 6, such as to provide for automatic offset cancellation.
An ADC 416 converts the analog voltage signal to a corresponding digital
signal that represents the sensor signal. Thus, for the example of blood
pressure monitoring, the ADC provides a digital signal indicative of the
sensed blood pressure.

[0058]A combiner 418 combines the digital outputs from the power sensor
410 and the ADC 416. For instance, the combiner 418 can be implemented as
a two-channel combiner and parity generator that combines the respective
signals for transmission as a digital signal via a transmitter 420. The
transmitter 420 can be a frequency-shift keyed transmitter that generates
a wireless signal 424 via an antenna 422, although other types of
transmitters could be used. Thus, the wireless digital signal 424
contains both sensor information as well as the RF power feedback
information. The wireless signal 424 can be received at an external
receiver and used to adjust the power level at the external source 402.

[0059]FIG. 9 is a timing diagram illustrating the operation of the
adaptive power control system in the context of the system 400 of FIG. 8.
In the example of FIG. 9, a coupling factor k is plotted with respect to
time along with signals representing power coupled to the implant, the
power level signal (e.g., from the power sensor 410), and the external
input power. As mentioned above, the external input power is adjusted as
a function of the power sensing data, as represented by the RF power
feedback information in the signal 424.

[0060]As illustrated in FIG. 9, when k decreases, the power coupled to the
implant temporarily drops, causing the power data bit to remain low. The
adaptive controlling program (e.g., implemented in the external RF
powering system) steps up the external input power to regain the coupled
power in the implant system to a desired level. In this example, the
transmitting RF power can be controlled by adjusting a supply voltage of
a class-E amplifier (PA) implemented as the external power source 402.
Thus, when k increases, the external input power is stepped down to a
proper level resulting in lower external power consumption, thus a
reduced RF input power. When k is constant (e.g., moveable system or
implant not moving), the power data bit alternates between a high and low
states, causing a steady power with ripple at the step up/down frequency.

[0061]FIG. 10 depicts an example of a blood pressure monitoring system
implanted in an animal in accordance with an embodiment of the invention.
The system implanted can be configured according to one or any
combination of features shown and described herein.

[0062]What have been described above are examples and embodiments of the
invention. It is, of course, not possible to describe every conceivable
combination of components or methodologies for purposes of describing the
invention, but one of ordinary skill in the art will recognize that many
further combinations and permutations of the present invention are
possible. Accordingly, the invention is intended to embrace all such
alterations, modifications and variations that fall within the scope of
the appended claims. In the claims, unless otherwise indicated, the
article "a" is to refer to "one or more than one."

[0063]Although specific embodiments of the invention have been described
and illustrated, the invention is not to be limited to the specific forms
or arrangements of parts so described and illustrated. The scope of the
invention is to be defined by the claims appended hereto and their
equivalents.